Field of the Invention
The present invention broadly relates to an optical displacement measuring apparatus. More particularly, the present invention relates to an optical displacement measuring apparatus making use of the Doppler effect to determine the displacement of, for example, a moving object or fluid (hereinafter referred to as "moving object") and measure the velocity of the moving object, without contacting the moving object. The apparatus operates by irradiating light onto the moving object and detecting Doppler shifts in the frequency of the irradiated light scattered by the moving object in accordance with the velocity of the moving object.
2. Description of the Related Art
Laser Doppler speed meters have conventionally been used as apparatuses for accurately measuring the velocity of a moving object without contacting the moving object. The laser Doppler speed meter measures the velocity of a moving object by using the Doppler effect in which the frequency of a laser beam scattered by the irradiated moving object shifts in proportion to the velocity of the moving object.
FIG. 1 is a schematic view showing the main portion of an example of a laser Doppler speed meter disclosed in Japanese Unexamined Patent Publication No. 4-230885. Referring to FIG. 1, reference numeral 101 denotes a laser Doppler speed meter, reference numeral 1 denotes a laser beam, reference numeral 2 denotes a collimator lens, reference numeral 7 denotes a moving object to be measured, reference numeral 10 denotes a diffraction grating with a grating pitch d, and reference numerals 11 and 12 denote convex lenses each having a focal length f. When the distance from the diffraction grating 10 to the lens 11 is a, and the distance from the lens 12 to the object 7 to be measured is b, distances a and b satisfy the relationship a+b=2f.
The laser beam with a wavelength .lambda. of about 0.68 .mu.m is emitted from the laser diode 1 and converted into a parallel light beam 3 with a diameter of 1.2 mm by the collimator lens 2. The parallel light beam 3 impinges upon the transmission-type diffraction grating 10 with a grating pitch d of 3.2 .mu.m in a direction vertical to the direction of orientation of the grating 10. Here, .+-. first order diffraction light beams 5a and 5b diffracted in accordance with Formula (1) set forth below leave the diffraction grating 10 at a diffraction angle .theta.=12.degree.. EQU d sin .theta.=.lambda. (1)
The light beams 5a and 5b, which impinge upon a convex lens 11 with a focal length f=15 mm, are converted into light beams 13a and 13b, as shown in FIG. 1. The light beams 13a and 13b, which impinge upon the other convex lens 12 separated from the convex lens 11 by a distance of 2f (in this embodiment, 2f=30 mm), are reconverted into parallel light beams designated 14a and 14b, which irradiate the object 7 to be measured, moving in a direction indicated by arrow 7a at a velocity V (in mm/sec) as 1.2-mm diameter spots at an angle equal to the diffraction angle .theta. at which the light beams are diffracted.
The convex lens 12 and the converging lens 8 cause the light beams scattered by the object 7 to be measured to effectively converge onto a light receiving portion 9a of a photodetector 9, which detects a Doppler light signal with a frequency given by Formula (2): ##EQU1## where a=10 mm, and b=20 mm, so that b is relatively large. Therefore, the speed meter can be set with greater freedom at various locations by increasing the working distance.
In such a construction, when the laser beam wavelength .lambda. of the laser diode 1 changes due to temperature changes or other factors, the diffraction angle .theta. changes in accordance with Formula (1), but from Formula (2) the Doppler signal ultimately does not depend on the laser beam wavelength .lambda., so that the laser Doppler speed meter provides excellent environmental stability.
The laser Doppler speed meter of FIG. 1 is constructed such that the diffraction angle of the light from the grating 10 is equal to the diffraction angle of the light at the object to be measured. Therefore, even when the laser wavelength changes, the Doppler signal does not change, as a result of which highly accurate speed measurements can be achieved. However, it has been necessary, in recent years, to further efficiently increase the working distance.
FIG. 2 is a schematic view showing the main portion of a laser Doppler speed meter constructed in such a way that the diffraction angle does not equal the intersection angle. It has the same construction as that of the speed meter of FIG. 1, except that an afocal optical system, formed by lenses 11 and 12 with different focal lengths, is used. In this example, light beams 5a and 5b leave diffraction grating 10 at a diffraction angle .theta.1.
Here, the frequency of the Doppler signal is given by Formula (3): EQU F=2V sin .theta.2/.lambda. (3)
where .theta.2 is the incident angle at which parallel light beams 15a and 15b impinge on object 7.
FIG. 3 is a view showing an afocal optical system whose lenses have different focal lengths. The first lens 11 with a focal length of f1 and the second lens 12 with a focal length of f2 are separated by a distance equal to f1+f2. The distance from the diffraction grating 10 to the first lens 11 is a, while the distance from the second lens 12 to the object 7 to be measured is b. In FIG. 3, a light beam I is formed when a=f1 and b=f2, while a light beam II is formed when a.noteq.f1 and b.noteq.f2, with a=f1-.DELTA.a and b=f2+.DELTA.b. The relationship between .DELTA.a and .DELTA.b is given by Formula (4): EQU b=(f2/f1).sup.2 .DELTA.a (4)
From the Formula, the working distance is made larger, without increasing the size of the laser Doppler speed meter itself, by increasing the f2/f1 ratio and decreasing a.
However, a further decrease in a in such an optical system with a relatively small f1 compared to f2 increases aberration, since the light beams impinge upon the outer periphery of the second lens, thereby making it difficult to provide a large measurement depth, while preserving high accuracy.
In addition, the use of an afocal optical system determines to a certain extent the arrangement of the optical system, thus making it difficult to further increase the working distance, while further decreasing the size of the optical system.